1. Field of the Invention
The present invention relates to the utilization of certain acrylic copolymers as dispersants for aqueous drilling muds, and, more especially, relates to novel drilling muds comprised of such acrylic copolymeric dispersants which are more particularly suitable for drilling at medium and great depths, in regions which are complicated by difficulties related to the temperature of the ground, to the substantial content of solids with which the mud may become charged over the course of the drilling operation and, finally, to environmental constraints in respect of toxicity.
2. Description of the Prior Art
It is known to the art that, when drilling natural gas or petroleum/oil wells via rotary process, a drilling fluid is used, the purpose of which is, in particular, to cause the excavated material or waste which is formed at the cutting edges to ascend in the well towards the surface. That fluid, which is introduced under pressure by way of the axis of the drill cord, passes through the head of the drill bit and then ascends through the annular space defined by the drill cord and the drill hole itself, at the same time performs other functions. notably those of cooling the head of the bit, rendering fluid-tight the walls of the well and balancing the internal pressures by means of hydrostatic pressure.
Aqueous drilling fluids or drilling muds typically comprise an aqueous suspension containing various materials which are intended to impart thereto certain physical and chemical properties.
In particular, they are as follows:
(i) viscosifying or viscosity modifying agents: water-insoluble swelling clays which are selected in dependence upon the proportion of electrolytes in the water, often associated with one or more viscosifying polymers;
(ii) a dispersant or thinning agent;
(iii) optionally, a fluid loss additive, such as carboxymethylcellulose or low molecular weight of starches;
(iv) optionally, a weight-providing agent, such as barytes.
The water may be soft water, sea water or a brine.
One of the principal problems which confronts this art is that of providing a system which has stable rheological characteristics (viscosity, yield value, thixotropy) throughout the entirety of the drilling cycle, and notwithstanding stop-pages in circulation which may be necessary. As the drilling operation progresses and the waste material ascends toward the surface, the high content of solid contaminants causes the increase in viscosity, and pressure in the circulatory system tends to become excessively high if aggregation and/or flocculation of the particles of clay occurs.
In order to reduce viscosity, or at least to limit the increase thereof and to deflocculate the clay, viscosity reducing agents (dispersants or thinning agents) are commonly added to the drilling fluid.
Relatively high temperatures may be encountered in petroleum wells as a result of the increase in the depth thereof. It is, therefore, essential for the additives to have adequate thermal stability. For obvious reasons of economy, it is desirable for them to be effective in small amounts.
Polyphosphates are not a very attractive alternative as drilling and dispersants, because of their instability at temperatures of higher than 70.degree. C. Derivatives of lignite and lignosulfonates are less susceptible to thermal degradation. However, the effectiveness of lignite is limited by electrolyte contaminating species (sodium chloride and calcium salts in particular) which effect precipitation thereof. Chromium and ferrochrome lignosulfonates are used both as dispersants and fluid loss additives. They are effective in the presence of calcium salts and at temperatures of up to 150.degree. C., but their toxicity due to the chromium content seriously and increasingly limits the use thereof.
Of the other dispersants which are recommended for drilling muds, exemplary are polymers such as homopolymers of acrylic acid, having very low molecular weight (U.S. Pat. No. 3,764,530) and copolymers of maleic acid and sulfonated styrene (U.S. Pat. No. 3,730,900).